The invention takes relates to a hydraulic control arrangement which is used for a mobile working machine, in particular for a wheel loader.
It is known from DE 39 09 205 C1 to damp the pitching vibrations of wheel loaders which occur, in particular, with the loading shovel full and at relatively high traveling speed, using a damping system which is a constituent part of the hydraulic control arrangement of the wheel loader. For the purpose of vibration damping, the generally two hydraulic lifting cylinders for raising and lowering the loading shovels can be connected via a shut-off valve to a hydraulic accumulator which can be charged to a limit pressure by a hydraulic pump, via a filling line which branches off from the pump line upstream of the directional control valve block and in which a filling valve is located. The shut-off valve, which is arranged between the hydraulic accumulator and the lifting cylinders, is closed as long as the loading shovel is working, and can be opened by the driver or automatically as soon as pitching vibrations occur during travel or as soon as the traveling speed is above a certain value, e.g. above 6 km/h.
The hydraulic fluids can then flow back and forth freely between the lifting cylinders and the hydraulic accumulator, with the result that the loading shovel is no longer connected rigidly to the vehicle body and the pitching vibrations are damped. Relative movement between the loading shovel and vehicle body makes it possible for the pressures occurring in the lifting cylinders and the hydraulic accumulator to be high enough to shorten the service life of the hydraulic accumulators. The hydraulic accumulator is also subjected to very high pressure loading when, with the shut-off valve open, the directional control valve assigned to the lifting cylinders is actuated and the lifting cylinders are extended to their full extent.
Another damping system against pitching vibrations, which is likewise part of the hydraulic control arrangement of a working machine, is known from DE 41 29 509 C2. In this case, the filling line branches off from a working line which runs between the lifting cylinders and the directional control valve assigned thereto. The shut-off valve arranged in the filling line is, at the same time, the filling valve and pressure-controlled and can be opened by the load pressure, prevailing in the working line, of the lifting cylinders counter to the accumulator pressure, which acts on a rear control chamber on the valve element of the shut-off valve, and counter to the force of a weak compression spring. The accumulator pressure is thus in each case only slightly lower than the highest load pressure of the lifting cylinders which occurs during a working cycle. In order to damp the pitching vibrations, the rear control chamber of the shut-off valve is relieved of loading in relation to the tank via a pilot valve, with the result that the shut-off valve opens and hydraulic fluids can be pushed back and forth freely between the hydraulic accumulator and the lifting cylinders.
In a development of the hydraulic control arrangement according to DE 41 29 509 C2, it is known from DE 196 08 758 A1 to arrange, upstream of said pilot valve, a second, pressure-controlled pilot valve which, provided the load pressure in the lifting cylinders and in the working line does not exceed a predetermined limit pressure, passes on the accumulator pressure, and when the limit pressure is exceeded passes on the load pressure, to the first pilot valve. Thus, when the first pilot valve is located in its rest position, that is to say when the damping system against pitching vibrations is not switched on, the rear control chamber is subjected to the action of the accumulator pressure when the load pressure is lower than the limit pressure and is subjected to the action of the load pressure when the load pressure is higher than the limit pressure. In the latter case, the shut-off valve closes, with the result that the hydraulic accumulator is protected against pressures exceeding the limit pressure. If, however, the rear control chamber is connected to the tank via the first pilot valve, then it is also the case with the hydraulic control arrangement according to DE 196 08 758 A1 that the protection of the hydraulic accumulator is no longer successful. The latter may be subjected to the action of pressures which far exceed the limit pressure and are caused by the relative movement between the loading shovel and the vehicle body. It is likewise possible for a very high pressure to occur in the hydraulic accumulator when, with the first pilot valve in operation and the shut-off valve thus open, the directional control valve assigned to the lifting cylinders is actuated and the lifting cylinders are moved against a stop.
The object of the invention is thus to develop further a hydraulic control arrangement of the above mentioned type, such that, with the condition under which the shut-off valve can be moved into its through position fulfilled, e.g. at a higher traveling speed than 6 km/h or following actuation of an electric switch by the driver or in the case of a certain lifting height of the loading shovel, the hydraulic accumulator is also protected against pressures which are severely detrimental to the service life.
This object is achieved, in the case of a hydraulic control arrangement of the above-mentioned type, wherein the shut-off valve can also be moved, with the first condition fulfilled, into its shut-off position when the load pressure in the pressure chamber of the hydraulic cylinder or the accumulator pressure reaches a maximum pressure. In this way, pressures which are above the maximum pressure are also kept away from the hydraulic accumulator when a xe2x80x9cdamping system onxe2x80x9d signal is present. In this case, according to another feature of the invention, the maximum pressure is advantageously selected to be higher than the limit pressure. This does not yet stress the hydraulic accumulator excessively since, when the damping system is activated or in the standby state, the number of changes between pressure increase and pressure decrease, the absolute level of the changes in pressure and the change in pressure over time are generally lower than during an operating cycle during which time a material is received or displaced by the loading shovel.
According to another feature of the invention hydraulic control arrangement has a shut-off valve which assumes a shut-off position under the action of a spring and, by activation of an actuating element, can be switched over into the through position. In a particularly straightforward manner, it is provided, then, that the actuating element is deactivated when the load pressure or the accumulator pressure reaches the maximum pressure. If the shut-off valve is actuated, for example, by an electromagnet, then the load pressure or the accumulator pressure can be sensed by an electric pressure sensor which emits a signal when it establishes the maximum pressure. The signal is passed onto an electric control unit which switches off the electromagnet. Of course, the pressure sensor may also be formed by a pressure switch located directly in the power circuit of the electromagnet. The pressure sensor or pressure switch can easily be adjusted to a maximum pressure which is higher than the limit pressure.
Alternatively, the shut-off valve can be switched over by an actuating piston, which acts counter to the activated actuating element and a spring and is subjected to the action of the load pressure or the accumulator pressure, from the through position into the shut-off position when the load pressure or the accumulator pressure reaches the maximum pressure.
Advantageously, the shut-off valve is also the filling valve and is precontrolled by a pilot-valve arrangement, with the result that its valve piston may be large and it is possible to open large flow cross sections and thus, with the damping system activated, to allow hydraulic-fluid exchange between the hydraulic cylinder and hydraulic accumulator without throttling. The valve piston of the shut-off valve can be forced in the opening direction by the load pressure and in the closing direction by a pressure prevailing in a control chamber and by a closing spring. The closing spring causes the accumulator pressure to be smaller than the load pressure in each case by a difference in pressure equivalent to the force of the closing spring. The control chamber of the shut-off valve can be connected, via the pilot-valve arrangement, to the hydraulic accumulator for the purpose of charging the hydraulic accumulator, to the pressure chamber of the hydraulic cylinder for the purpose of shutting off the hydraulic accumulator and to a tank for the purpose of flow taking place through the shut-off valve in a directionally independent manner.
As in the case of a directly actuated or else precontrolled shut-off valve which does not also have the function of filling valve at the same time, it is also possible, then, with a shut-off valve according to a feature of the invention for the actuating element of the pilot valve, in dependence on whose control position the control chamber of the shut-off valve can be subjected to load pressure or can be relieved of load in relation to the tank, to be deactivated when the load pressure or the accumulator pressure reaches the maximum pressure.
Alternatively, the pilot valve can be switched over by an actuating piston, which acts counter to the activated actuating element and a spring and is subjected to the action of the load pressure or of the accumulator pressure, from one position, in which flow can take place through the shut-off valve in a directionally independent manner, into another position, in which the shut-off valve is shut off, when the load pressure or the accumulator pressure reaches the maximum pressure. The spring acts on the actuating piston independently of the pilot-valve element which is to be operated, in order that it does not always attempt to move the same into a certain control position, and is prestressed for the purpose of predetermining the maximum pressure. A shut-off valve according to a feature of the invention can have a pilot-valve arrangement with two pilot valves. Another feature of the invention is a pilot-valve arrangement with a pilot valve with a spring-centered central position. In this case, the pilot valve also protects the hydraulic accumulator against excessive pressures during the charging operation, that is to say during the working cycle with the damping system not activated. This is because the pilot valve is moved by the actuating piston from the second control position into the third control position when a pressure corresponding to the prestressing of the spring acting on the actuating piston independently of the valve element which is to be operated is reached.
A feature of the invention provides a pilot-valve arrangement of the shut-off valve with two pilot valves which, nevertheless, are connected up to one another, and to the control chamber of the shut-off valve. The first pilot valve has a first connection, at which, depending on the position of a second pilot valve operated by activation of an actuating element, accumulator pressure or tank pressure is present, a second connection, at which load pressure is present, and a third connection, which is connected to the control chamber of the shut-off valve. In a first control position, which the first pilot valve assumes under the action of a spring, the first connection and the third connection are connected to one another and, in a second control position, into which the first pilot valve can be switched over by an actuating piston, which acts counter to a prestressed spring and is subjected to the action of the load pressure or the accumulator pressure, when the load pressure or the accumulator pressure reaches the limit pressure, the second connection and the third connection are connected to one another. The limit pressure is equivalent to the force of the prestressed spring. Without the prestressing being changed, the actuating piston is active, irrespective of the control position of the second pilot valve, whenever the limit pressure is reached. The charging pressure in the hydraulic accumulator cannot become higher than the limit pressure whether or not the damping system is activated. Maximum pressure and limit pressure thus correspond to one another. According to another feature of the invention the maximum pressure can be increased above the limit pressure in that the prestressing of the spring which acts counter to the actuating piston is changed, at the same time as the activation of the actuating element of the second pilot valve, from a value which corresponds to the limit pressure to a higher value. This advantageously takes place, in that the spring is supported on an adjustable prestressing piston which, upon activation of the actuating element, is subjected to the action of a pressure which displaces it with the effect of increasing the spring prestressing. If the directional control valve can be actuated hydraulically, then the maximum precontrol pressure is advantageously passed on to the prestressing piston in order to increase the spring prestressing.
In the case of a hydraulic control arrangement according to another feature of the invention, the spring acts on the actuating piston preferably via a valve piston of the first pilot valve. This renders a straightforward design possible.